Hard to see, but its spring steel formed around the wrenches, with a detent on the top side that captures the wrap around part.
Ive seen them on super cheap (junk) taskforce brand wrench sets at lowes, but would like a larger size and to be able to buy seperate. Might seem like an odd setup to most, but would be perfect for my work tool rig since I need a set of flex gearwrenches semi regulary.
The ones at lowes only hold 1/4-5/8 and I would like up to 3/4+